Currently, transparent conductive thin films are used for various applications in a variety of fields including photoelectronics, displays and energy industry.
In the area of light emitting devices, development of transparent conductive ohmic electrode structures, which perform functions of smooth hole injection and high-efficiency light emission, is now being actively undertaken throughout the world.
Transparent conductive oxides (TCOs) and transparent conductive nitrides (TCNs) are currently the most actively researched and studied of transparent conductive thin film materials.
The transparent conductive oxides (TCOs) include, for example, indium oxide (In2O3), tin dioxide (SnO2), zinc oxide (ZnO), and indium tin oxide (ITO). The transparent conductive nitrides (TCNs) include, for example, titanium nitride (TiN).
However, these materials exhibit a relatively high sheet resistance value and reflectivity and a relatively low work function. Therefore, using such materials alone, there is difficulty in application of such materials to p-type transparent ohmic electrodes of top-emitting gallium nitride-based light emitting devices.
To review specific problems associated with use of the above-exemplified thin film materials:
Firstly, since the above-mentioned transparent conductive thin films have high sheet resistance values close to about 100 Ω/unit area, upon forming thin films via Physical Vapor Deposition (PVD) methods such as sputtering, e-beam or heat evaporation, such large resistance characteristics result in difficulty to accomplish current spreading in the horizontal direction of the light emitting device (parallel to layer-layer interfaces) as well as difficulty of smooth hole injection in the vertical direction, thus making it difficult to embody high-brightness light emitting devices having large area and high capacity.
Secondly, since the above-mentioned transparent conductive thin films have high reflectivity and absorptivity for light emitted from gallium nitride-based light emitting diodes, luminous efficiency of diodes formed therefrom is low.
Thirdly, transparent conductive thin films such as indium tin oxide (ITO) and titanium nitride (TiN) have a relatively low work function and thus suffer from difficulty in formation of ohmic contact via direct contact with p-type gallium nitrides.
Finally, when they are applied as electrodes in direct ohmic contact with gallium nitride-based semiconductors, the transparent conductive oxides (TCOs) suffer from difficulty in formation of high quality ohmic contact electrodes due to production of gallium oxide (Ga2O3), an insulating material, on the surface of gallium nitride layer in the course of thin film formation processes, due to the fact that gallium (Ga) is a powerful oxidant.
Meanwhile, light emitting devices are broadly divided into top-emitting light emitting diodes (TLEDs) and flip-chip light emitting diodes (FCLEDs).
Currently, widely used top-emitting light emitting diodes are configured so as to emit light through an ohmic contact layer in contact with a p-type clad layer. However, in order to implement high-brightness top-emitting light emitting diodes, there is a need for a good-quality current spreading layer to compensate for the high sheet resistance value of the p-type clad layer having a low hole concentration. Therefore, it is necessary to provide functions of smooth hole injection and current spreading and excellent light emission by forming a current spreading thin film layer having a low sheet resistance value and high light-transmittance as the ohmic contact layer.
In top-emitting light emitting diodes known hitherto, the ohmic contact layer in which a nickel (Ni) layer and a gold (Au) layer are sequentially formed on the p-type clad layer is widely used.
Nickel-gold ohmic contact layers are known to have an excellent specific contact resistance of about 10−3 to 10−4Ω□ and are semi-transparent when annealed under oxygen (O2) atmosphere.
These conventional ohmic contact layers, when annealed at temperatures of about 500 to 600° C. under oxygen (O2) atmosphere, form nickel oxide (NiO), a p-type semi-conductor oxide, between island-like gold layers and on the upper parts thereof, at an interface between gallium nitride constituting the p-type clad layer and the nickel layer applied as the ohmic contact layer, which results in decreased Schottky barrier height (SBH) and thereby easy supply of dominant carrier holes around the surface of the p-type clad layer.
In addition, it is understood that annealing following formation of the nickel-gold layer structure on the p-type clad layer removes Mg—H intermetallic complexes, thus leading to an effective carrier concentration at the surface of the p-type clad layer of more than 1018 via a reactivation process that increases concentration of magnesium dopants on the surface of gallium nitride which in turn causes inversion of tunneling between the p-type clad layer and nickel oxide-containing ohmic contact layer, thereby providing ohmic conductivity exhibiting low specific contact resistance.
However, the top-emitting light emitting diode utilizing a semi-transparent ohmic contact layer in the form of a nickel-gold structure contains gold (Au) inhibiting light-transmittance and therefore has low luminous efficiency, thus limiting realization of next generation high-capacity, high-brightness light emitting devices.
In addition, even though research and development of flip-chip light emitting diode structures, which emit light through a transparent sapphire substrate by application of a p-type reflective ohmic electrode in order to increase luminous efficiency of light, are now being undertaken, there is difficulty in realization of high quality flip-chip light emitting diodes, due to the absence of an electrically, mechanically and thermally stable p-type reflective ohmic electrode.
In order to overcome limitations exhibited by such top-emitting and flip-chip light emitting diodes, research into utilization of transparent conductive oxides, from which gold (Au) is completely excluded so as to have light-transmittance superior to the semi-transparent nickel (oxide)-gold layer structure that is conventionally used as the p-type ohmic contact layer, for example ITO, are reported in a variety of literature [IEEE PTL, Y. C. Lin, etc. Vol. 14, 1668; and IEEE PTL, Shyi-Ming Pan, etc. Vol. 15, 646]. Recently, realization of the top-emitting light emitting diodes exerting more improved output power compared to conventional nickel-gold structures, via use of ITO ohmic contact layers, is reported in Semicond. Sci. Technol., C S Chang et al, 18 (2003), L21. However, such structures of ohmic contact layers are capable of augmenting luminous efficiency of light emitting devices, but still suffer from problems associated with a relatively high operation voltage, thereby presenting a great deal of limitations in application thereof to high-brightness light emitting devices having a large area and high capacity. On the other hand, U.S. Pat. No. 6,287,947 discloses a method for preparing a light emitting diode having improved light-transmittance and electrical properties via use of an oxidized thin nickel-gold or nickel-silver structure in conjunction with indium tin oxide (ITO). However, the method proposed in the above US Patent has drawbacks such as complicated processes for forming the ohmic contact electrode, and high sheet resistance of ohmic electrodes formed of transition metals including nickel metal or oxides of Group II elements of the Periodic Table and thus difficulty to realize high-efficiency light emitting devices.
Further, oxidized transition metals including nickel exhibit disadvantages of lowered light-transmittance.
There is a great deal of difficulty in development of high quality p-type ohmic electrodes having excellent electrical and optical properties as discussed above, and underlying causes of such difficulty may be summarized as follows:
1. Low hole concentration of p-type gallium nitride and thereby a high sheet resistance value of more than 104 Ω/unit area.
2. Difficulty of smooth hole injection in the vertical direction due to formation of high Schottky barrier height and width at interfaces between p-type gallium nitride and electrodes resulting from the absence of a highly transparent electrode material having a relatively high work function as compared to that of p-type gallium nitride.
3. Reciprocal relationship between electrical properties and optical properties in most materials, and consequently, generally high sheet resistance values of transparent electrodes having high light-transmittance, thereby resulting in sharp decline of current spreading in the horizontal direction.
4. Deterioration in electrical properties of light emitting devices due to production of insulative gallium oxide (Ga2O3) on the surface of the gallium nitride layer in the course of direct deposition of the transparent conductive thin film layer on the upper part of the p-type gallium nitride.